The role of bone graft materials in clinical applications to aid the healing of bone has been well documented over the years. Most bone graft materials that are currently available, however, have failed to deliver the anticipated results necessary to make these materials a routine therapeutic application in reconstructive surgery. Improved bone graft materials for forming bone tissue implants that can produce reliable and consistent results are therefore still needed and desired.
In recent years intensive studies have been made on bone graft materials in the hopes of identifying the key features necessary to produce an ideal bone graft implant, as well as to proffer a theory of the mechanism of action that results in successful bone tissue growth. At least one recent study has suggested that a successful bone tissue scaffold should consider the physicochemical properties, morphology and degradation kinetics of the bone being treated. (“Bone tissue engineering: from bench to bedside”, Woodruff et al., Materials Today, 15(10): 430-435 (2012)). According to the study, porosity is necessary to allow vascularization, and the desired scaffold should have a porous interconnected pore network with surface properties that are optimized for cell attachment, migration, proliferation and differentiation. At the same time, the scaffold should be biocompatible and allow flow transport of nutrients and metabolic waste. Just as important is the scaffold's ability to provide a controllable rate of biodegradation to compliment cell and/or tissue growth and maturation. Finally, the ability to model and/or customize the external size and shape of the scaffold is to allow a customized fit for the individual patient is of equal importance.
Woodruff, et. al. also suggested that the rate of degradation of the scaffold must be compatible with the rate of bone tissue formation, remodeling and maturation. Recent studies have demonstrated that initial bone tissue ingrowth does not equate to tissue maturation and remodeling. According to the study, most of the currently available bone graft implants are formulated to degrade as soon as new tissue emerges, and at a faster rate than the new bone tissue is able to mature, resulting in less than desirable clinical outcomes.
Other researchers have emphasized different aspects as the core features of an ideal bone graft implant. For example, many believe that the implant's ability to provide adequate structural support or mechanical integrity for new cellular activity is the main factor to achieving clinical success, while others emphasize the role of porosity as the key feature. The roles of porosity, pore size and pore size distribution in promoting revascularization, healing, and remodeling of bone have long been recognized as important contributing factors for successful bone grafting implants. Many studies have suggested an ideal range of porosities and pore size distributions for achieving bone graft success. However, as clinical results have shown, a biocompatible bone graft having the correct structure and mechanical integrity for new bone growth or having the requisite porosities and pore distributions alone does not guarantee a good clinical outcome. What is clear from this collective body of research is that the ideal bone graft implant should possess a combination of structural and functional features that act in synergy to allow the bone graft implant to support the biological activity and an effective mechanism of action as time progresses.
Currently available bone graft implants fall short of meeting these requirements. That is, many bone graft implants tend to suffer from one or more of the problems previously mentioned, while others may have different, negatively associated complications or shortcomings. One example of such a graft implant is autograft implants. Autograft implants have acceptable physical and biological properties and exhibit the appropriate mechanical structure and integrity for bone growth. However, the use of autogenous bone requires the patient to undergo multiple or extended surgeries, consequently increasing the time the patient is under anesthesia, and leading to considerable pain, increased risk of infection and other complications, and morbidity at the donor site.
When it comes to synthetic bone graft substitutes, the most rapidly expanding category consists of products based on calcium sulfate, hydroxyapatite and tricalcium phosphate. Whether in the form of injectable cements, blocks or morsels, these materials have a proven track record of being effective, safe bone graft substitutes for selected clinical applications. Recently, new materials such as bioactive glass (“BAG”) have become an increasingly viable alternative or supplement to natural bone-derived graft materials. In comparison to autograft implants, these new synthetic implants have the advantage of avoiding painful and inherently risky harvesting procedures on patients. Also, the use of these synthetic, non-bone derived materials can reduce the risk of disease transmission. Like autograft and allograft implants, these new artificial implants can serve as osteoconductive scaffolds that promote bone regrowth. Preferably, the graft implant is resorbable and is eventually replaced with new bone tissue.
Many artificial bone grafts available today comprise materials that have properties similar to natural bone, such as implants containing calcium phosphates. Exemplary calcium phosphate implants contain type-B carbonated hydroxyapatite whose composition in general may be described as (Ca5(PO4)3x(CO3)x(OH)). Calcium phosphate ceramics have been fabricated and implanted in mammals in various forms including, but not limited to, shaped bodies and cements. Different stoichiometric implants, such as hydroxyapatite (HA), tricalcium phosphate (TCP), tetracalcium phosphate (TTCP), and other calcium phosphate (CaP) salts and minerals have all been employed in attempts to match the adaptability, biocompatibility, structure, and strength of natural bone. Although calcium phosphate based materials are widely accepted, they lack the ease of handling, flexibility and capacity to serve as a liquid carrier/storage media necessary to be used in a wide array of clinical applications. Calcium phosphate materials are inherently rigid, and to facilitate handling are generally provided as part of an admixture with a carrier material; such admixtures typically have an active calcium phosphate ingredient to carrier volume ratio of about 50:50, and may have a ratio as low as 10:90.
As previously mentioned, the roles of porosity, pore size and pore size distribution in promoting revascularization, healing, and remodeling of bone have been recognized as important contributing factors for successful bone grafting. Yet currently available bone graft implants still lack the requisite chemical and physical properties necessary for an ideal graft implant. For instance, currently available graft implants tend to resorb too quickly (e.g., within a few weeks), while some take too long (e.g., over years) to resorb due to the implant's chemical composition and structure. For example, certain implants made from hydroxyapatite tend to take too long to resorb, while implants made from calcium sulfate or β-TCP tend to resorb too quickly. Further, if the porosity of the implant is too high (e.g., around 90%), there may not be enough base material left after resorption has taken place to support osteoconduction. Conversely, if the porosity of the implant is too low (e.g., 10%,) then too much material must be resorbed, leading to longer resorption rates. In addition, the excess material means there may not be enough room left in the residual graft implant for cell infiltration. Other times, the graft implants may be too soft, such that any kind of physical pressure exerted on them during clinical usage causes them to lose the fluids retained by them.
Accordingly, there continues to be a need for better bone graft implants. For instance, it would be desirable to provide improved bone graft implants offering the benefits just described, and in a form that is even easier to handle and allows even better clinical results. Embodiments of the present disclosure address these and other needs.